1. Field of Invention
The present invention relates to a LNT (Lean NOx Trap) control method for a vehicle, and more particularly, a technology of implementing a rich mode to be able to improve performance of purifying NOx of LNTs.
2. Description of Related Art
A LNT (Lean NOx Trap) is a purifying device that adsorbs NOx in an exhaust gas and reduces the adsorbed NOx into N2 using a reducer such as HC and CO in a rich exhaust gas in a rich mode, which is a rich combustion state.
However, the whole NOx separated from LNT in the rich mode described above is not reduced and some of NOx is exhausted, and this phenomenon is called slip of NOx and the ratio of NOx reduced from the LNT, not being slipped, is called NOx conversion ratio.
The slip of NOx is significantly influenced by the NOx conversion ratio which varies according to the temperature of the LNT. FIG. 1 exemplifies a NOx conversion ratio as a function of LNT temperature and it can be seen from the figure that the NOx conversion ratio increases with the increase in temperature, in the section of 200˜350° C., but the NOx conversion ratio decreases with the increase in temperature in the section of 350° C. or above.
It can be seen from FIG. 1 that it is effective to perform a rich mode at the position where 50% or more can be converted, when NOx adsorbed to LNT is reduced to N2; otherwise, a large amount of NOx slips without being reduced to N2 in the rich mode.
In the related art, when the amount of NOx adsorbed to LNT increases, it enters a rich mode, reduction of NOx is started, and then the amount of NOx adsorbed to the LNT becomes zero or the lambda sensor values at the front and rear ends of the LNT become the same or substantially the same, it is determined that it is because the entire NOx adsorbed to the LNT has been reduced, and the rich mode is ended.
As described above, controlling a rich mode of a LNT does not consider a purification rate of NOx as a function of the temperature of the LNT, therefore when it enters the rich mode in a region with a low LNT temperature, NOx separated from the LNT and slipping increases.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.